Video stream modifier

ABSTRACT

A compressed data stream modifier  270  is disclosed. The stream modifier modifies an input data stream that may comprise audio and video data streams conforming to any one of the well-known video compression standards, for example, MPEG or AVC. The input stream is first de-multiplexed to obtain a single video elementary stream. The video elementary stream is then decoded by means of a variable-length decoder and provided to the data stream modifier. A decoding complexity of each frame is estimated by an estimator  210  and provided to a controller  250 . The controller selects a number of regions from each frame based on some psycho-visual criteria, such that a method of modification of these regions can bring the decoding complexity within an available capacity at a decoder. An efficient means of adapting a decoding complexity to an available capacity is thus obtained.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a 371 National Phase of PCT patent application No.PCT/IB2005/054316, filed Dec. 19, 2005, and claims benefit/priority ofEuropean patent application No. 04106886.7, filed Dec. 22, 2004.

BACKGROUND

1. Field

The present disclosure relates to systems and methods of modifying acompressed data stream to be decoded at a decoder.

2. Description of Related Art

Video information, typically comprising a sequence of picture frames,can be compressed to form a digital data stream for storage andtransmission. Some of the compression standards that have attainedwidespread use for compressing and decompressing video information arethe Moving Pictures Expert Group (MPEG) Standards for video encoding anddecoding, in particular MPEG-2. MPEG Standards are used for variousapplications of storage, transmission and display of video information,such as optical storage systems, digital versatile disc (DVD)technology, and digital television (DTV) broadcasts. While decoding aMPEG data stream, the decoding complexity may vary from frame to frame.It may occur that the decoding complexity is higher than a capacityavailable at a decoder, as a decoder might generally not be equipped todeal with peak loads. In such cases, the decoder will be unable todecode the complete picture. Consequently, the output picture mayabruptly suffer from quality degradation. In order to adapt the decodingcomplexity of a compressed data stream of video information to theavailable capacity at a decoder and to obtain an optimum picture qualityat the output, several solutions have been suggested in the prior art.

A method and system for decoding a stream of video data with a modifieddecoding process at the decoder resulting in a dynamic adaptation ofcomplexity is known from U.S. Pat. No. 6,631,163. This patent describesa method of dynamically setting a threshold value for a scalable modulewithin a MPEG-2 video decoder system. One of the embodiments of thisprior art comprises a scaling application that selectively decodes onlythose bi-directionally predictable blocks that meet a certain thresholdand to discard those bi-directionally predictable blocks that do notmeet the threshold. The threshold is dynamically adapted to changingresource levels and to fluctuating input data that occur in a system. Anadjusted threshold value based on a first group of pictures (GOP) isused for decoding a second GOP.

In the prior art mentioned above, a threshold value is dynamically setfor a scalable module within a decoder system. Estimating the complexityand modifying a method of decoding at the decoder is a delayed reaction.Often, region dropouts occur in essential parts of the picture,resulting in annoying artifacts to the viewer. Moreover, using athreshold value based on a previous GOP to decode a current GOP couldresult in distortion of unexpected regions in the output picturesequence. In other words, the prior-art method noticeably degrades avideo output quality.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the innovations herein will be elucidated with reference tothe implementations and embodiments described hereinafter and withreference to the accompanying drawings. The Figures are schematicrepresentations of one of many examples of embodiments of the invention,wherein:

FIG. 1 schematically shows a functional block diagram illustratingcomponents of a general video compression and decompression system.

FIG. 2 schematically shows a functional block diagram illustratingcomponents of one example of an embodiment of a compressed data streammodifier according to the present invention.

FIG. 3 schematically shows a flow chart illustrating steps of a methodof modifying a compressed data stream according to the presentinvention.

FIG. 4 schematically shows a representation of an embodiment of a videocompression system according to the present invention.

FIG. 5 schematically shows a representation of an embodiment of avideo-processing apparatus according to the present invention.

DETAILED DESCRIPTION OF EXEMPLARY IMPLEMENTATIONS

Generally, the present invention relates to devices and methods fordynamically adapting a compressed data stream corresponding to a picturesequence to an available capacity of a decoder. Embodiments of thepresent invention are capable of analyzing a compressed data stream,estimate a decoding complexity in terms of peak memory and bandwidthrequirements and adapt a method of decoding indicated in the stream inorder to reduce the complexity of the data stream itself to fall withinthe capacity available at a decoder. In one of the embodiments, acompressed data stream is modified in such a way that regions ofbi-directional pictures can be predicted with a single referencepicture, thereby reducing the memory bandwidth by half the originalbandwidth. In another embodiment, a method of decoding is modified for aselected set of regions of bi-directional pictures, the selection beingmade in accordance with a set of psycho-visual criteria of theseregions.

Various illustrative aspects of the innovations herein are describedbelow, followed by a detailed description of the drawings.

The invention relates to a device for modifying a compressed data streamto be decoded at a decoder, the compressed data stream being obtained byencoding a sequence of pictures comprising regions of pixels, the devicecomprising:

-   -   estimating means for estimating a decoding complexity required        for decoding a picture from the compressed data stream; and    -   modifying means for modifying the compressed data stream when        the decoding complexity of the picture approaches a capacity of        the decoder.

The invention further relates to a video-processing system forcompression and decompression of a sequence of pictures comprising sucha device.

The invention further relates to a video-processing apparatus comprisingsuch a device.

The invention further relates to a method of modifying a compressed datastream to be decoded at a decoder, the compressed data stream beingobtained by encoding a sequence of pictures comprising regions ofpixels, the method comprising the steps of

-   -   estimating a decoding complexity required for decoding a picture        from the compressed data stream; and    -   modifying the stream when the decoding complexity of the picture        approaches a capacity of the decoder.

The invention further relates to a computer program product comprising acode enabling a processor to execute a method of modifying a compresseddata stream to be decoded at a decoder, the compressed data stream beingobtained by encoding a sequence of pictures comprising regions ofpixels, the product comprising:

-   -   a code for estimating a decoding complexity required for        decoding a picture from the compressed data stream; and    -   a code for modifying the stream when the decoding complexity of        the picture approaches a capacity of the decoder.

It is an object of the present invention to adapt the decodingcomplexity of a compressed data stream to an available capacity of avideo decoder with an improved video output quality.

The object of the invention is achieved in that the modifying means ofthe device for modifying a compressed data stream as mentioned in theopening paragraphs are arranged to modify a method of decoding indicatedin the compressed data stream.

The device proposed by the invention modifies a method of decodingindicated in a compressed data stream. The device estimates the decodingcomplexity from the compressed data stream. The device is designed toact only when the estimated complexity approaches a capacity availableat the decoder. The device has a priori knowledge of the availablecapacity at the decoder. There are several ways in which the device canacquire this knowledge. The device preferably takes a pre-emptive actionwhen the decoding capacity approaches the complexity. When thecomplexity estimate approaches a 100%-capacity, the device can bring thedecoding complexity within limits. For instance, the modificationprocess, preferably the amount of modification for relatively complexparts of an image increases as the complexity gets closer to or exceeds100% of decoder capacity. This ensures a smooth, gradual and pre-emptivereduction of the decoding load. An optimal picture quality is obtainedfor a fixed decoder capacity. For any fixed decoder capacity, an optimalpicture quality can be obtained under all circumstances and for anycompressed data stream.

The decision to modify the compressed data stream can be based on thecurrent picture and the action can be effective on the same picture. Asa result, the picture quality at the output is expected to be betterthan in the prior-art method, in which the decision to modify a currentpicture sequence is based on the previous picture sequence.

A prior-art modification of the method of decoding at the decoder stageis usually too late and important parts of a picture may be affected. Ifthe decoder is unable to assign resources, blank regions or artifactswill appear in crucial portions of the output picture. It is anadvantage of the present invention to bring the decoding complexity ofeach picture within the available complexity, simultaneously resultingin a better video output quality. An optimum picture quality at theoutput may be ensured for an available capacity at the decoder.

In a preferred embodiment of the device modifying the compressed datastream, the decoding complexity is based on the computational load orthe memory access bandwidth for decoding the picture.

A decoding complexity of encoded video data may be estimated from one ormore of a number of values, for example, a number of arithmeticinstructions and/or a number of memory read/write operations. Thesevalues may in turn be derived directly from a number of parametersavailable in a compressed data stream, for example, a number of DCTcoefficients to be processed, a position of a macroblock in the picture,a reference to (none, single or double) anchor frames and/or aprediction vector length for motion prediction. A decoder may getoverloaded particularly due to two values, namely computational load andmemory access bandwidth. In a preferred embodiment of the invention, acomplexity estimate is derived directly from the compressed data stream.Consequently, modification of the method of decoding indicated in thestream is aimed at a reduction of computational load or memory access.

A preferred embodiment of the device modifying the compressed datastream is characterized in that the modifying means are arranged toselect a region on the basis of a heuristic psycho-visual selectioncriterion and modify a method of decoding for that region.

The device according to the invention is preferably arranged to firstselect at least one region based on a heuristic psycho-visual selectioncriterion. It is also possible to select at least one region randomlyfrom the picture so as to simplify the process of selection. However, aselection based on certain heuristic rules is preferred because of thefollowing advantages.

While selecting the regions for modification of the decoding method,psycho-visual criteria can be advantageously applied so that the imagequality at the output is not noticeably degraded. A criterion ispreferably identified in such a way that artifacts are least annoying toor less noticeable by a visual system model of an observer watching themodified decoded pictures. By applying such a criterion, a number ofregions may be chosen so as to minimize the possibility of theirdistortion being noticed by the human visual system.

A suitable selection criterion is, for example, the proximity of theregion to boundaries of the picture.

Distortion of a region at or near the boundaries of a picture is lessnoticeable to the human visual system as compared to that of a region inthe central part of the picture.

A further suitable selection criterion is, for example, the speed ofmovement of the region.

A motion vector preferably represents the speed of movement of a region.When the motion is fast, i.e. the motion vector is large, distortion inthe corresponding regions of movement may be less noticeable as comparedto regions having little motion. Thus, regions of fast movement are moresuitable for reducing the decoding complexity and may be moreadvantageously selected for modification of decoding methods as comparedto regions of slower movement.

A further suitable selection criterion is, for example, a regionrepresenting a complex texture.

Regions with coarse textures are preferable candidates for toleratingrelatively more distortion than regions with smooth textures and, hence,they can be preferably selected for modification of the decoding method.

Preferably, the modifying means are arranged to modify a method ofmotion-compensated prediction of a region of a picture frombi-directional prediction to unidirectional prediction. More preferably,regions selected by applying certain psycho-visual selection criteriaare subjected to modification of motion-compensated prediction.

By changing the method of motion-compensated prediction frombi-directional to unidirectional, only a single reference picture needsto be accessed instead of two reference pictures in the case ofbi-directional prediction. The memory access for regions to be decodedby applying bi-directional prediction can thus be advantageouslyreduced.

Also, the modifying means are arranged to modify a quantized transformcoefficient present in the compressed data stream.

A preferred way of modifying a quantized transform coefficient is torender the transform coefficients of higher frequency components tozero. Changing the quantization steps from fine to coarse canalternatively effect modification of transform coefficients. Themodification of the quantization step may be implemented as a stepchange or a gradual change.

The modifying means are preferably arranged to select a region ofrectangular shape.

Regular shapes of squares and rectangles conform to MPEG standards, forexample, blocks or macroblocks or slices according to MPEG-2 standards.In the device according to a preferred embodiment, macroblocks asspecified by MPEG-2 standards can be handled in a particularly efficientmanner.

The modifying means are preferably arranged to select a region of ashape that adapts to an object in the picture.

The device is preferably equipped to handle regions of irregular shapesto conform with more recent MPEG standards, for example, MPEG-4 and AVC.In such cases, multiple regions or a single region may correspond to asingle object present in the picture. Alternatively, a region maycorrespond to multiple objects in the picture. This is useful becausesome regions of a picture get more viewer attention than others. Forexample, a region comprising a human face receives more attention by theviewer than a region comprising a background.

It is a further object of the invention to provide a video-processingsystem for compression and decompression of a sequence of pictures, ofthe type described in the opening paragraphs, which provides arelatively efficient means of adapting the compressed data stream to theavailable complexity at the decoder. A device according to theinvention, in particular in one of the preferred embodiments as set outabove, will also be referred to as ‘a preparser’ hereinafter.

The video-processing system includes a preparser according to theinvention for modifying the compressed data stream. The architecture ofa decoder does not need to be altered for co-operating with thepreparser, resulting in a flexible and efficient system for any numberof decoders. A preparser according to the invention and a decoder areseparated from each other. A preparser is capable of functioning with orwithout an input regarding the available load capacity at the decoder.As a consequence, a preparser can be advantageously located at multiplelocations. A preparser may form part of a data compressor or of a datadecompressor, or it may be located anywhere within a video distributionsystem, thereby increasing its versatility. A preparser can function ina stand-alone mode or can be integrated within a complete system. Apreparser can be realized fully either in hardware, software or anycombination of hardware and software.

A video-processing apparatus, capable of one or more of the followingtypes of video-related processing can advantageously include a preparserfor:

-   -   Video decoding, encoding and transcoding;    -   Video displaying;    -   Video recording; and    -   Video content analysis and content recognition.

A video-processing apparatus may comprise additional units such as areceiving unit, a processing unit and/or a display unit. Avideo-processing apparatus may be, for example, a digital television, aset-top box, a personal video recorder, a digital versatile disk (DVD)player or recorder, a Blue-Ray player or recorder, a satellite tuner,head-end equipment of a cable service provider, a video analysis systemor a video summarizing system.

The preparser is designed to receive a compressed video streamcompatible with e.g. a MPEG or AVC standard and output a compressedvideo stream compatible with the same standard. Though the stream ismodified, it still maintains full compatibility with the standard. Inother words, the preparser can be designed to be capable of receiving acompressed video stream from an encoder compatible with MPEG standardsand also capable of outputting a compressed video stream to a decodercompatible with MPEG standards. Thus, the preparser may be adapted towork with existing encoders and decoders.

It is a further object of the invention to provide a method defined inthe opening paragraphs, which provides a relatively efficient means ofadapting a compressed data stream to the available complexity at adecoder.

This object of the invention is achieved in that the step of modifyingthe compressed data stream comprises modifying a method of decodingindicated in the stream, for example, modification of the referencenumber referring to the number of anchor frames to be considered whiledecoding.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

These and other aspects of the compressed data stream modifier, theimage-processing apparatus, and a method according to the invention areapparent from and will be elucidated with reference to theimplementation and embodiment described hereinafter and with referenceto the accompanying drawings.

FIG. 1 schematically shows a functional block diagram illustratingcomponents of a general video compression and decompression system. Avideo compression system 110 comprises a video encoder 120. The encoder120 receives a picture sequence 1 from an external source. The picturesequence can also be generated internally, for example, in DVD players.The output signal 121 of the image compression system comprises acompressed data stream conforming to one of the video compressionstandards, for example, the MPEG-2 standard. The output signal 121 canbe transmitted through a wired or wireless media and received at a videodecompression system 130 comprising a decoder 140. The decoder input isa compressed data stream 1 and the decoder output 141 comprises apicture sequence that is substantially similar to the original picturesequence 1.

FIG. 2 schematically shows a functional block diagram illustratingcomponents of one embodiment of a compressed data stream modifier 270according to the present invention. An input data stream may comprisemultiple audio and video data streams. The input stream is firstde-multiplexed to obtain a single video elementary stream (ES) by meansof well-known techniques. A single video ES can be preferably receivedand processed by an embodiment of the device according to the invention,as shown in FIG. 2. A video ES conforming to any one of the well-knownvideo compression standards, for example, MPEG or AVC is received at theinput of a variable-length decoder (VLD) 200.

The output of the VLD is a data stream 201 that can be analyzed forestimating a decoding complexity. An estimator 210 estimates a decodingcomplexity on a frame-by-frame basis from the data stream 201. Anestimator of a simple form, for example, can count the number ofbi-directionally predicted regions and estimate the memory bandwidthrequired at the decoder for that particular frame. An estimator of amore complex form can estimate a complexity measure based on a number ofparameters, for example, a number of DCT coefficients to be processed,total computational load and memory bandwidth. An estimated complexityvalue 212 is made available to controller 250. Additional data onpredictability of regions and their motion information can be madeavailable to the controller 250. Additional header data available in thedata stream, for example, video format, bit rate, quantization levels,field/frame coded, and the number of reference pictures for predictionof each region can also be made available to the controller. Thecontroller can preferably maintain an array for each region of apicture. Elements of this array can preferably relate to variouscharacteristic of the region, for example, its position, motion vector,number of reference pictures, etc. These data can also be used as inputto a decision rule based on yet another psycho-visual criterion.

The controller 250 may vary from a simple system to a complex system ina variety of predetermined embodiments of the invention. In an exampleof a simple embodiment, the controller 250 is equipped to receive anestimate of memory bandwidth, compare it with an available decoderbandwidth, select a number of bi-directionally predicted regions andsend the identification of selected regions to a modifying means 230. Inan example of a complex embodiment, the controller 250 comprises meansfor identifying and selecting regions by means of a knowledge-basedsystem. Knowledge in the form of psycho-visual criteria can act upon thedata of each region supplied by the estimator 210. Knowledge in the formof rules can be built in the controller 250. Regions are selected insuch a manner that the artifacts or distortion due to modification ofthe method of decoding are less perceptible to the human visual system.Rules can be preferably based on the location of the region in thepicture, the speed of movement of the region and/or the texture contentof the region. A measure of texture can also be estimated by estimator210 and made available to the controller 250. Location information of aselected number of regions is conveyed to modifying means 230. It ispossible to design and build controller 250 with varying degrees ofcomplexities in terms of amount of data stored for each region, numberof rules in the knowledge base, and decision mechanism. In addition,controller 250 can decide upon the method of modification of the datastream and convey this command to the data stream modifier 210. A methodof modification of the data stream may be, for example, a change ofbi-directional prediction of the region to unidirectional predictionand/or modification of quantized transform coefficients. The output ofcontroller 250 is in the form of command instructions 251 comprisinglocation information of regions selected for modification and method ofmodification of each region conveyed to a modifying means 230.

An additional knowledge base 260 can have knowledge of the decodingcapacities of decoder systems known to the data stream modifier 270.Alternatively, the stream modifier through an external input 2 canreceive information regarding the available capacity of a decoder.

The modifying means 230 can also receive a compressed data stream 211,sufficiently delayed by a delaying means 220, for example, a delay lineor a memory to compensate for the processing delay in controller 250.Thus, command instructions 251 are automatically synchronized with atime-delayed data stream 221. The modifying means 230 modifies themethod of decoding the time-delayed data stream 221 in accordance withthe command instructions 251. The modified data stream 231 is receivedat the input of a variable-length encoder (VLE) 240. The output 241 ofthe VLE is a modified video ES, fully complying with the compressionstandards of the original data stream.

In one of the embodiments, estimator 210 estimates the memory bandwidthrequired for decoding at the decoder. If the bandwidth approaches anavailable capacity at a decoder, controller 250 will select a subset ofregions from a set of bi-directionally predicted regions. The controller250 will further send instructions to the stream modifying means 230 formodifying the method of decoding for the selected set of regions frombi-directional prediction to unidirectional prediction. The memorybandwidth requirements may thus be reduced by as much as half theoriginal bandwidth requirements. Another embodiment stores a set ofrules of psycho-visual criteria in the knowledge base 260. Thecontroller 250 may then obtain the applicable rules via link 261 andselects a set of regions for modification of the decoding method, basedon the set of these rules. Various other embodiments can be realized byvarying the degree of complexity of each components estimator,controller, stream modification means or stream-delaying means and alsoby combining each or several of these variations thereof.

FIG. 3 schematically shows a flow chart illustrating steps of a methodof modification of a compressed data stream according to the presentinvention. A video elementary stream 1 (ES) corresponding to a sequenceof pictures is received in a first step 300 and variable-length decodingaccording to one of the well-known methods is performed. The decodingcomplexity of a frame is estimated in step 310. It is compared with acapacity of the decoder in step 320. If the complexity is within thedesired limits of the decoding capacity, the stream will not bemodified. In that case, steps 330, 340 and 350 are bypassed and the nextpicture frame will be considered as demonstrated in steps 360 and 300.

If the complexity approaches or overruns the capacity, preventive actionin the form of modification of the decoding method will be carried outin steps 330, 340 and 350. In order to avoid a sudden or abrupt loss ofquality of output images, corrective action may start, for example, whenthe complexity estimate reaches 80% of the decoder capacity. The amountof correction needed may gradually increase when the estimatedcomplexity reaches 90% and crosses over to more than 100%. In step 330,regions for which the modification of the decoding method is to becarried out are selected on the basis of one or more psycho-visualcriteria. A number of regions are so selected as to bring the decodingcomplexity within a desired range. A type of modification to beperformed is decided in step 340, for example, specifying frombi-directional prediction to unidirectional prediction. The modificationof the decoding method is carried out in step 350. The process continuesuntil the end of the picture sequence is reached, which is checked bystep 360. The process comes to an end in step 370.

In one of the embodiments, the memory bandwidth needed for decoding atthe decoder is estimated in step 310. Then, in step 330, a set ofregions of bi-directional pictures for which the method of predictionwill be modified from bi-directional prediction to unidirectionalprediction is selected. Preferably, in step 330, also a set ofpsycho-visual criteria such as the proximity of the region to theboundaries of the picture, the speed of movement of the region, thedirection of movement of the region and the texture content of theregion can be used in deciding for which regions a decoding methodshould be modified. A number of such embodiments are possible bychoosing varying degrees of complexities for each functional step.

FIG. 4 schematically shows a representation of an embodiment of a videocompression system according to the present invention, comprising:

-   -   A compression system 400 for compressing a sequence of pictures        1;    -   A device 410 for modifying a compressed data stream; and    -   A decompression system 420 for decompressing the modified data        stream.

The video compression system receives a sequence of pictures andassociated audio from a video source. The data compression system 400reduces the volume of data by encoding the picture sequence by applyingwell-known techniques. The output of the compression system comprises acompressed data stream 401 conforming to one of the well-knownstandards, for example, MPEG or AVC. A number of such compressed datastreams can be combined to obtain a single data stream.

The video steam modifier 410 according to an embodiment of the inventionreceives the compressed data stream 401. The data stream 401 accordingto the MPEG standard can be de-multiplexed by means of a de-multiplexerto obtain an elementary video stream (VS) and an audio stream. Theelementary video stream is subjected to modification of the decodingmethod as described with reference to FIGS. 2 and 3. After modification,the modified compressed data stream 411 preferably still conforms to thestandards of the original elementary video stream. The modified datastream 411 can be multiplexed with a corresponding audio stream toobtain the data stream compatible with the original output of thecompression system.

The modified data stream 411 can be decompressed by means of adecompression system 420 to obtain a picture sequence 421. The streammodifier 410 can be located at a video distribution system, isolatedfrom the compression or decompression system, for example, at a head-endof a local service provider.

FIG. 5 schematically shows a representation of an embodiment of avideo-processing apparatus 500 according to the present invention. Acompressed stream modifier 510 according to the present inventionreceives a signal 501 comprising a compressed data stream and renders anoutput data stream 511 compatible with the standards of the originaldata stream 501. The data stream 511 can be decoded at a decoder 520.The output of decoder 520 comprises a picture sequence 521 which may bedisplayed on a display device. Alternatively, the data stream can bere-transmitted through a wired or wireless distribution system orrecorded on a recording media. A compressed data stream modifieraccording to the invention can be made versatile and located in one ofmany possible locations to provide video distribution service to anapparatus, for example, at a central computer of a home network or at ahead-end of a distribution point of a mobile communication network.

In the claims, use of the verb “comprise” and its conjugations does notexclude the presence of a plurality of elements or steps. In the deviceclaims enumerating several means, several of these means can be embodiedby one and the same item of hardware. The invention can be implementedby means of hardware comprising several distinct elements and by meansof a suitably programmed computer. In practice, the algorithmiccomponents disclosed in this text may be realized (entirely or in part)as hardware (e.g. parts of an application-specific IC) or as softwarerunning on a special digital signal processor, or a generic processor,etc.

A computer program product should be understood to be any physicalrealization of a collection of commands enabling a processor-generic orspecial purpose, after a series of loading steps (which may includeintermediate conversion steps, such as translation to an intermediatelanguage and a final processor language) to get the commands into theprocessor to execute any of the characteristic functions of theinvention. In particular, the computer program product may be realizedas data on a carrier such as a disk or tape or program code on paper.Apart from the program code, characteristic data required for theprogram may also be embodied as a computer program product.

Some of the steps required for the operation of the method may alreadybe present in the functionality of the processor instead of in thedescription of the computer program product, such as data input andoutput steps.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention. Apart from combinations of elements ofthe invention as combined in the claims, other combinations of theelements are possible. Any combination of elements can be realized in asingle dedicated element.

1. A device for modifying a compressed data stream to be decoded at adecoder, the compressed data stream being obtained by encoding asequence of pictures comprising regions of pixels, the devicecomprising: estimating means for estimating a decoding complexityrequired for decoding a picture from the compressed data stream; andmodifying means for modifying the compressed data stream when thedecoding complexity of the picture approaches a capacity of the decoder,characterized in that the modifying means are arranged to modify amethod of decoding indicated in the compressed data stream; wherein themodifying means are arranged to select a region on the basis of aheuristic psycho-visual selection criterion and modify a method ofdecoding for said region.
 2. A device as claimed in claim 1, wherein thedecoding complexity is based on a computational load or a memory accessbandwidth for decoding the picture.
 3. A device as claimed in claim 1,wherein the selection criterion is a proximity of the region toboundaries of the picture.
 4. A device as claimed in claim 1, whereinthe selection criterion is a speed of movement of the region.
 5. Adevice as claimed in claim 1, wherein the selection criterion is ameasure of texture of the region.
 6. A device as claimed in claim 1,wherein the modifying means are arranged to modify a method ofmotion-compensated prediction of a region of a picture frombi-directional prediction to unidirectional prediction.
 7. A device asclaimed in claim 1, wherein the modifying means are arranged to modify aquantized transform coefficient present in the compressed data stream.8. A device as claimed in claim 1, wherein the modifying means arearranged to select a region of rectangular shape.
 9. A device as claimedin claim 1, wherein the modifying means are arranged to select a regionof a shape that adapts to an object in the picture.
 10. Avideo-processing system for compression and decompression of a sequenceof pictures, the system comprising: an encoder for encoding the sequenceof pictures and output of a compressed data stream; a device formodifying the compressed data stream according to claim 1; and a decoderfor decoding the modified compressed data stream to obtain a sequence ofpictures.
 11. A video-processing apparatus comprising receiving meansfor receiving a signal comprising a compressed data stream correspondingto a sequence of pictures; and a device for modifying the compresseddata stream according to claim
 1. 12. A method of modifying a compresseddata stream to be decoded at a decoder, the compressed data stream beingobtained by encoding a sequence of pictures comprising regions ofpixels, the method comprising: estimating, via means for estimating, adecoding complexity required for decoding a picture from the compresseddata stream; and modifying, via means for modifying the compressed datastream, the stream when the decoding complexity of the pictureapproaches a capacity of the decoder, characterized in that modifyingthe compressed data stream, via the means for modifying, comprisesmodifying a method of decoding indicated in the stream; wherein themodifying means are arranged to select a region on the basis of aheuristic psycho-visual selection criterion and modify a method ofdecoding for said region.
 13. A method as claimed in claim 12, whereinthe decoding complexity is based on a computational load or a memoryaccess bandwidth for decoding the picture.
 14. A method as claimed inclaim 12, wherein the selection criterion is a proximity of the regionto boundaries of the picture, a speed of movement of the region, or ameasure of texture of the region.
 15. A method as claimed in claim 12,wherein the modifying means are arranged to modify a method ofmotion-compensated prediction of a region of a picture frombi-directional prediction to unidirectional prediction.
 16. A method asclaimed in claim 12, wherein the modifying means are arranged to modifya quantized transform coefficient present in the compressed data streamor arranged to select a region of a shape that adapts to an object inthe picture.
 17. A device for modifying a compressed data stream to bedecoded at a decoder, the compressed data stream being obtained byencoding a sequence of pictures comprising regions of pixels, the devicecomprising: an estimating component that estimates a decoding complexityrequired for decoding a picture from the compressed data stream; and amodifying component that modifies the compressed data stream when thedecoding complexity of the picture approaches a capacity of the decoder,characterized in that the modifying component is arranged to modify amethod of decoding indicated in the compressed data stream; wherein themodifying component is arranged to select a region on the basis of aheuristic psycho-visual selection criterion and modify a method ofdecoding for said region.
 18. A device as claimed in claim 17, whereinthe decoding complexity is based on a computational load or a memoryaccess bandwidth for decoding the picture.
 19. A device as claimed inclaim 17, wherein the selection criterion includes a proximity of theregion to boundaries of the picture, a speed of movement of the region,or a measure of texture of the region.
 20. A device as claimed in claim17, wherein the modifying component is arranged to modify a quantizedtransform coefficient present in the compressed data stream or arrangedto select a region of a shape that adapts to an object in the picture.